Many of modern machines, especially machine tools in which machine-side spindles including main spindles are rotated at high speed, make use of a drive system in which the machine-side spindles are connected directly to a motor shaft to be rotated thereby. In directly connecting a machine-side spindle and the motor shaft, the connecting operation is carried out by interposing a coupling between them. If centering between the machine-side spindle and the motor shaft is not exact, however, vibration is caused, thus exerting a bad influence on the working accuracy, shortening the life of the machine, or increasing the possibility of production of noises.
In the case where a spindle-through coolant is used, moreover, there is a possibility of the coolant leaking if the machine-side spindle and the center of the motor shaft are not aligned.
There are two known methods for centering the machine-side spindle and the motor shaft. In one method, center misalignment between the spindle and the shaft is measured by means of a measurer attached to the machine-side spindle as adjustment operation is performed to correct the attachment position of the motor. In the other method, the respective center positions of the spindle and the shaft are compulsorily aligned by means of a socket that constitutes a coupling.
According to the former method, however, it is difficult to secure a working space wide enough for measurement operation if the construction of the machine is complicated. In the case of a structure such that the machine-side spindle and the motor shaft are externally closed up tight when the motor is mounted, for example, in the case of a spindle structure of a machine tool or the like, external measurement operation cannot be carried out without regard to the presence the working space.
In the case of the latter method, on the other hand, the center misalignment is not actually measured during the adjustment operation, although the operation itself is easy. Therefore, the tolerance for the center misalignment should be restricted within the limit of about 30 microns at the most, depending on the rigidity of the machine itself and the working accuracy of the socket. This accuracy is not always high enough for spindles that rotate at high speed.